Social media is a stock market sector that gets Justin Bieber levels of fawning from all sorts in the investment world.

So why is it so hard to get flows into an ETF devoted to the category. Consider the tribulations of Groupon, or of Facebook for that matter, writes Brendan Conway of Barron's.

Conway notes recent travails of the sector such as Groupon's firing of 32-year-old chief executive Andrew Mason, following poor quarterly results and a 24% Thursday stock plunge, as well as Facebook's cooling stock rebound.

He also looks at the struggles of he year-and-a-half-old Global X Social Media Index ETF, which according to XTF.com has less than $12 million in assets.

Juste as important, Conway notes that there are fewer than 30 stocks in this ETF. "Even getting to that number requires a more liberal definition of “social media” than you might think. That’s for the simple reason that there aren’t yet enough public companies doing the sorts of things that Groupon and Facebook do," he writes.